Evidence Implicating Calcium as the Second Messenger for Red Pigment-Concentrating Hormone in the Prawn Palaemonetes pugio

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Abstract
Three different agents were examined for their effect on pigment concentration in the large erythrophores of Palaemonetes pugio: guanosine 3',5' cyclic monophosphate (cGMP), ouabain, and the divalent cation ionophore A23187. High concentrations (10 mM) of cGMP in the medium did not cause pigment concentration. High concentrations (1 mg/ml) of ouabain failed to inhibit pigment concentration induced by either the ionophore A23187 or the red pigment-concentrating hormone. The ionophore A23187, in Ca⁺⁺-containing saline, caused pigment concentration in the large erythrophores of three internal organs: ventral nerve cord, intestine, and ovary. When erythrophores whose pigment had been concentrated by the ionophore were exposed to Ca⁺⁺-free/EGTA-containing saline, pigment dispersion resulted. Erythrophores whose pigment was concentrated by long-term exposure to a white background did not disperse their pigment when treated with the ionophore in Ca⁺⁺-free/EGTA saline. The red pigment-concentrating hormone responded to Ca⁺⁺-free/EGTA saline by an initial concentration of pigment, followed by redispersion. The activity of the red pigment-concentrating hormone was greatly inhibited in saline whose calcium had been replaced with lanthanum. On the basis of the data above, it is tentatively proposed that the red pigment-concentrating hormone, upon binding to its receptor, behaves as a Ca⁺⁺ ionophore.

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